The present invention relates to an improved process for the production of diesters of phosphoric acid 2,5-dioxo-4,4-diphenyl-imidazolidin-1-ylmethyl ester, which are key intermediates in the preparation of 5,5-diphenyl-[(3-phosphonooxy)methyl]-2,4-imidazolidinedione disodium salt (Cerebyx.RTM., also known as fosphenytoin sodium) described in U.S. Pat. Nos. 4,260,769 and 4,925,860, which are herein incorporated by reference. Cerebyx.RTM. is useful as an anticonvulsant, antiepileptic, and antiarrhythmic agent.
A synthetic procedure for preparing 3-(hydroxymethyl)-5,5-diphenylhydantoin dibenzyl phosphate ester is disclosed in Varia S. A., et al., Journal of Pharmaceutical Sciences, 1984;73:1068-1073. The aforementioned procedure requires the use of silver dibenzyl phosphate. This reagent is expensive, light sensitive, and silver byproducts are difficult to remove. Thus, special procedures are required to purify the desired product.
Metal salts of dialkyl phosphates have been used for phosphorylation of alkyl halides, and in general, the cation of choice is silver (Sasse K., "Methoden der Organisation Chemis" (Houben-Weyl), Band XLI/2, Thieme Verlag, Stuttgart, 1964:302-306). Precipitation of silver halide from the reaction mixture drives the reaction to completion. Salts of dialkyl phosphates with sodium or potassium as the gegenion have been used with some substrates, but are generally considered to be very poor nucleophiles for reaction with alkyl halides (Khorana H. G., "Some Recent Developments in the Chemistry of Phosphates of Biological Interest," John Wiley & Sons, New York, N.Y., 1961:13-14; Zwierak A, and Kluba M., Tetrahedron, 1971;27:3163-3170; U.S. Pat. Nos. 2,494,126, 2,494,283, and 2,494,284). In point of fact, Zwierzak A. and Kluba M., Tetrahedron, 1971;27:3163-3170 disclose that sodium or potassium salts of dialkyl phosphates are too unreactive to give satisfactory phosphorylation.
We have surprisingly and unexpectedly found that alkali metal phosphate esters react with either 3-(chloromethyl)- or 3-(bromomethyl)-5,5-diphenyl-2,4-imidazolidinedione to afford diesters of phosphoric acid 2,5-dioxo-4,4-diphenyl-imidazolidin-1-ylmethyl ester. The object of the present invention is an improved, short, efficient, and economical process that can be carried out on a manufacturing scale for the preparation of diesters of phosphoric acid 2,5-dioxo-4,4-diphenyl-imidazolidin-1-ylmethyl ester. Thus, the present process avoids the use of costly and unstable reagents such as silver phosphate esters and associated silver by-products, which are difficult to remove and is amenable to large-scale synthesis.